Lung cancer results in more deaths than colon, breast and prostate cancer combined. Conventional cytotoxic therapy of lung cancer is limited by side effects, and is rarely curative. Mechanism based therapies directed at tumor specific pathways offers hope for the development of novel treatments. With this in mind, we have studied activation of mammalian development pathways in human lung cancer to provide insights into how such interventions can be achieved. The morphogen sonic hedgehog (Shh), which signals to adjacent embryonic cells to specify morphogenic patterns and progenitor cell fates, is essential for lung development. In extensive preliminary studies, we provide compelling evidence that many human lung cancers activate Hedgehog (Hh) signaling. We demonstrate cell autonomous Hh signaling in small cell lung cancer (SCLC), whereas non-SCLC (NSCLC) sends a Shh signal to adjacent stromal cells. Moreover, we find that specific inhibition of Hh signaling by the Veratrum alkaloid cyclopamine inhibits the growth of SCLC cells exhibiting pathway activation both in vivo and in vivo. Although NSCLC cells express Shh, they are not sensitive to cyclopamine and do not demonstrate cell autonomous pathway activation in vivo. However, NSCLC cell lines which signal to adjacent fibroblasts in vivo are growth inhibited by cyclopamine in vivo, suggesting that tumor-stromal interactions mediated by Shh promote malignant growth. These data show that activation of the Shh pathway promotes the malignant behavior of lung cancer, and that inhibition of this pathway may represent a novel mechanism-based therapy. Outside of studies in CNS tumors, this is the first direct demonstration of Hh pathway activation in any human cancer. Moreover, our studies show that this phenomenon is not a general feature of carcinomas, but is restricted to epithelial systems in which Hh signaling plays a role in development. We propose to establish inhibitors of Hh signaling as clinically useful therapies in lung cancer using an approach integrating human tumor tissue arrays, molecular and cell biology studies, mouse models and basic pharmacology. First, we will identify the prevalence of Shh pathway activation in lung cancer and pre-malignant airway tissue using immunohistochemical markers. Then, using genetically engineered reporter cell lines and mouse models, we will study the pharmacologic effect of Hh pathway inhibitors on tumor growth, pathway activation and tumor-stromal interactions. Using these preclinical models, we will then perform delivery, dosing and toxicity studies as a rational basis for eventual phase one studies in humans. This research plan will firmly establish the importance of Shh signaling in lung cancer, and provide a rational, mechanism based approach for the treatment of lung cancer with inhibitors of the Shh pathway.